1. Field of the Invention
The present invention relates to production methods of an electroconductive mayenite compound having high electron density.
2. Description of the Related Art
Mayenite compounds have a typical composition expressed by 12CaO.7Al2O3, and have a characteristic crystal structure including three-dimensionally connected voids (cages) of approximately 0.4 nm in diameter. A frame forming these cages is positively charged, and forms 12 cages per unit lattice. One sixth of the cages are internally occupied by oxygen ions so as to satisfy the condition that the crystal is electrically neutral. These oxygen ions inside the cages, however, have a chemical characteristic different from other oxygen ions forming the frame. Therefore, in particular, the oxygen ions inside the cages are referred to as free oxygen ions. Mayenite compounds are also expressed as [Ca24Al28O64]4+.2O2− (Non-Patent Document 1).
A mayenite compound is provided with electrical conductivity when part or all of free oxygen ions inside the cages of the mayenite compound are replaced by electrons. This is because the electrons included in the cages of the mayenite compound are not so much confined to the cages and are freely movable inside the crystal (Patent Document 1). Such mayenite compounds having electrical conductivity are referred to as “electroconductive mayenite compounds” in particular.
Such electroconductive mayenite compounds may be produced by, for example, a production method by which a mayenite compound powder is put in a lidded carbon container and is subjected to heating at 1300° C. in a nitrogen gas atmosphere (Patent Document 2). Hereinafter, this method is referred to as Conventional Method 1.
Furthermore, electroconductive mayenite compounds may be produced by, for example, a production method by which a mayenite compound along with aluminum is put in a lidded alumina container and is subjected to heating at 1300° C. in a vacuum (Patent Document 2). Hereinafter, this method is referred to as Conventional Method 2.